Electrosurgery involves applying relatively high voltage, radio frequency (RF) electrical power to tissue of a patient undergoing surgery, for the purpose of cutting the tissue, coagulating or stopping blood or fluid flow from the tissue, or cutting and coagulating the tissue simultaneously. The high voltage, RF electrical power is created by an electrosurgical generator, and the electrical power from the generator is applied to the tissue from an active electrode manipulated by a surgeon during the surgical procedure.
The amount and characteristics of the electrosurgical power delivered to the patient is determined by the surgeon and depends on the type of surgical procedure to be performed and the amount of electrosurgical output power required, as well as the tissue characteristics of the patient. Selecting the cutting mode of operation causes the electrosurgical generator to continuously deliver relatively high RF power of moderate voltage. Selecting the coagulation mode of operation causes the electrosurgical generator to repetitively deliver relatively short bursts of high voltage, resulting in a relatively low average output power delivery. Selecting the “blend” mode of operation causes the electrosurgical generator to deliver output power having characteristics which are related to both cutting and coagulation. The blend mode of operation involves repetitively delivering relatively longer bursts of somewhat lower voltage RF output power, resulting in a relatively moderate average output power delivery. In the cut mode, for example, the continuous power output may be as high as 300 watts with an open circuit output voltage in the neighborhood of 2,000 volts peak to peak. In the coagulation mode, the bursts may reoccur at a frequency of approximately 30 kHz, have a time duration of approximately 3 microseconds, and have a peak to peak voltage of approximately 10,000 volts. A typical blend mode will involve bursts at the same frequency of approximately 30 kHz, but with time duration of approximately 5-7 microseconds and at a peak to peak voltage of approximately 4,000 volts. The higher voltage required for coagulation and blend is necessary to cause arcs of electrical power to jump from the active electrode to the tissue. Lower output voltage is used for cutting because electrical arcing is not as important or necessary for cutting.
The electrosurgical generator should also have the capability to deliver these types of RF electrosurgical power under a wide variety of different and rapidly changing output conditions. The impedance of the tissue into which the RF output power is delivered may change substantially from point-to-point as the active electrode is moved during the surgical procedure. For example, a highly fluid-perfused tissue such as the liver may exhibit a resistance or impedance in the neighborhood of 40 ohms. Other tissues, such as skin which has very little moisture content, or the marrow of bone because of its physiology, have impedance in the neighborhood of 1000-2000 ohms. Average tissue impedances range in the neighborhood of approximately 500 ohms, although the fat or adipose content of the tissue increases its impedance.
The power transfer or delivery capabilities of an electrosurgical generator, like any other power amplifier, depends on the output load characteristics into which the power is transferred. The maximum power transfer occurs when the internal impedance characteristic of the power amplifier is matched to the external impedance into which it delivers power. Since the internal impedance characteristic of the usual electrosurgical generator cannot be matched to the widely varying tissue impedance into which the electrosurgical power must be transferred, the electrosurgical generator should have the capability to deliver relatively higher amounts of power to compensate for the usual mismatch between the internal generator impedance and the widely varying values of the external tissue impedance, and to do so on an almost instantaneously changing basis as the surgeon moves through and works with the different types of tissues at the surgical site.
Further still, an electrosurgical generator must deliver the RF electrosurgical power under tightly regulated and precisely controlled conditions. Any attempt to meet the rapidly changing power requirements cannot be accompanied by excessive over-control to the point where the output RF electrosurgical power causes damage to the tissue or injury to the patient or surgical personnel. Rapid and reliable control over the delivered power is essential to safe and dependable performance of the surgical procedures.
Very few, if any, electrosurgical generators have the capability to meet all of these requirements, regardless of how well these requirements are understood. Indeed, almost no other electrical amplifier or power supply is subject to such widely varying requirements.
To deliver and regulate the RF electrosurgical output power, the typical electrosurgical generator uses a high voltage power supply to convert conventional commercial mains alternating current (AC) into direct current (DC) at a fixed voltage, and an RF amplifier output section which creates the RF electrosurgical power. The RF amplifier output section generates the RF output waveform, creates the bursts or duty cycle delivery of the RF waveform, and regulates the output power of the RF electrosurgical waveform delivered. It is typical that the high voltage power supply changes the amount of DC voltage delivered to the RF amplifier output section depending upon the mode of electrosurgical operation selected. For example, the high voltage power supply may deliver a DC output voltage of approximately 20-150 volts during the cut mode and approximately 50-300 volts during the coagulation mode.
The most prevalent type of RF amplifier output section used in an electrosurgical generator is a resonant circuit, in which a primary winding of an output transformer is connected to a capacitor to form the resonant circuit. Energizing pulses of electrical energy are delivered to the resonant circuit, and the resonant circuit responds to the energizing pulses by oscillating at a predetermined frequency established by the values of its inductance and capacitance. The transformer transforms the oscillations into the RF electrosurgical output waveform. The timing for the delivery of the energizing pulses creates either the continuous or the burst-like duty cycle delivery of the RF output waveform. The power or energy of the RF output waveform is controlled by the amount of power contained within each energizing pulse.
The amount of power contained in each energizing pulse is determined by the voltage of that pulse and the time width or on-time duration of the energizing pulse. The voltage of the energizing pulse is established by the high voltage power supply, because the DC output voltage from the high voltage power supply is used in creating the energizing pulse. Control over the on-time width of the energizing pulse is achieved by rapidly-responding digital logic circuits. Another type of RF output section sometimes used in electrosurgical generators is a switching circuit which switches energizing pulses of current from the DC power supply directly through the primary winding of the output transformer. The switching frequency establishes the frequency characteristic of the RF electrosurgical waveform. The amount of power delivered in the RF electrosurgical waveform is also related to energizing pulses switched through the primary winding. Again, the energy content of the switched energizing pulses is related to the voltage of each of those pulses and the time width, or on-time duration, of the switched energizing pulses. The responsiveness for power regulation purposes is therefore directly dependent upon the responsiveness of the switching circuitry which creates these energizing pulses.
Even though the typical electrosurgical generator will adjust the DC output voltage from the high voltage power supply according to the coagulation mode of operation selected, it is typical to require the RF amplifier and output section to perform all further regulation of the RF electrosurgical power delivered to the patient. To do so, the RF amplifier and output section primarily controls the on-time duration of the energizing pulses, for purposes of establishing output power regulation. Under very low or very high output power conditions, the time width or on-time duration of the energizing pulses may reach such small or large proportions of the overall cycle time that effective power regulation and conversion is difficult or impossible to achieve.
In both resonant circuit and switched RF amplifier output circuits, the optimum on-time for the energizing pulse is a 50% duty cycle, meaning that the on-time portion is one-half of the entire time of each cycle. As the on-time of the energizing pulses diminishes to a minimal portion of the overall cycle or as the on-time portion increases to a substantial portion of the overall cycle, the ability to regulate the output power becomes more difficult. A relatively short on-time portion of the energizing current pulse does not transfer a large amount of energy for conversion, making precise power regulation under low power delivery conditions with relatively short on-time energizing pulses more difficult. A relatively long on-time portion of the current pulse does not provide sufficient time during the off-time portion of each cycle for the energy to be converted, again making it difficult to regulate the amount of energy which is delivered under such circumstances. Thus, energizing pulses having a relatively short or a relatively long on-time do not provide the best power control and regulation capability. The optimal power regulation capability occurs when the on-time portion of each cycle of energizing pulses falls within a middle percentage of the entire cycle time.
Electrosurgical generators have typically used a type of high voltage power supply commonly known as a voltage mode DC power supply. In such a power supply, the voltage level of the supplied power is used as feedback for control and regulation purposes. A voltage mode DC power supply is relatively straightforwardly implemented by relatively inexpensive components. One of the disadvantages of a voltage mode DC high voltage power supply used in an electrosurgical generator is that it has a finite delay time when it is necessary to limit the current, or to shut down (i.e. turn off), or to rapidly ramp up, or increase, the DC output voltage. Because of the rigorous requirements for substantial variations in the RF electrosurgical power output and waveform, a voltage mode DC high voltage power supply limits the ability of the electrosurgical generator to adapt to changing tissue impedances and output power delivery and regulation circumstances.
Another type of DC power supply is commonly known as a switched current mode power supply. A current mode power supply controls the DC output voltage by controlling the amount of input current to the power supply. Because the input current can be rapidly controlled, a switched current mode power supply has the capability to respond very rapidly to changing output load conditions, and do so to a greater degree than a voltage mode DC power supply. The typical switched current mode power supply converts a source of coarsely regulated DC energy by switching pulses of input current from the coarsely regulated DC energy source through a primary winding of a conversion transformer. The energy from the pulses of input current flowing in the primary winding is transformed to the secondary winding and is then rectified. A conventional current mode controller controls the characteristics of the pulses of input current switched through the primary winding of the conversion transformer. The amount of current conducted by each pulse is sensed and fed back as a control signal to the current mode controller. The voltage of the energy converted from the conversion transformer is sensed and also fed back to the current mode controller. Based on these signals, the current mode controller generates switching signals for controlling the characteristic of pulses of input current. The current mode controller controls the time width of the pulses of input current to control the output power. The current mode controller is able to quickly adjust the pulses to increase or decrease the output voltage and the amount of power converted and transferred through the switched current mode power supply, or to cease generating the pulses altogether under extreme over-voltage or over-current conditions.
Adapting a switched current mode DC power supply to an electrosurgical generator creates difficulties not typically experienced in the typical use of a switched current mode DC to DC power supply. The leakage inductance in the conversion transformer interacts with the stray capacitance to cause the current pulses conducted through the primary winding to oscillate or “ring” at the beginning of each pulse. This ringing adversely affects the input current feedback signal and, unless suppressed, will cause the current mode controller to adjust the characteristics of the input current pulses under circumstances where no adjustment may be required or desirable, or even shut down the power conversion entirely. The typical current mode controller used in a current mode power supply has a built-in or inherit capability to suppress or “blank” an initial time portion of each input current feedback signal and thereby suppress the ringing.
However, in electrosurgical generators, the built-in blanking capability of the current mode controller is insufficient. In electrosurgical generators, electrical isolation of the generator from the conventional AC power mains is required as a safety measure, so that under a possible failure condition, electrical energy from the AC power mains does not feed through to the patient. This requires a conversion transformer having low-leakage current, typically resulting in high leakage inductance. These aspects of the conversion transformer exaggerate the ringing conditions in the input current feedback signal to the extent where the built-in blanking capability of a conventional current mode controller is not entirely satisfactory for use in a switched current mode power supply used in an electrosurgical generator.
The typical switched current mode power supply is intended for applications whose output voltage does not vary substantially as is required in electrosurgery. Additionally, blanking the initial portion of each switching signal is usually acceptable because of the relatively constant and non-varying load and power consumption conditions into which the typical current mode switched DC power supply delivers output power. However, under low output power conditions required for electrosurgical use, blanking an initial portion of an already shortened on-time of the feedback current signal may take up such a significant percentage of the feedback current signal that the remaining portion of the signal is insufficient for reliable and precise output power control and regulation.